The I region of the MHC encodes the Ia antigens or MHC class II genes, which are necessary for recognition of foreign antigen on antigen presenting cells and on B cells by helper T cells and therefore are crucial for initiation and regulation of an immune response. In the proposed studies we will use transgenic mice to dissect and analyze the different functions of MHC class II genes in the immune response. Modified MHC class II genes will be transferred into the mouse genome using suitable promoter sequences which will target the expression of these genes onto B cells, thymus and macrophages. Specific expression on B cells will be achieved by using immunoglobulin enhancer/promoter elements and on thymus by using 1.4 kb of 5' flanking sequences of the E alpha gene. To direct expression onto macrophages, we will first map the control elements determining cell type specific and inducible expression of glass II genes in this cell type by assaying the expression of the in vitro modified E alpha gene after transfection into macrophage lines. Parallel to these studies on cultured cells, the modified gene will be introduced into the germline of suitable recipient mice to study its regulation in vivo. The immune response capacity and properties of these transgenic mice will provide information on the biological function of these proteins associated with their expression in specific, appropriate cell types. Also, certain other aspects of class II gene function are approachable only through an animal model system: a current hypothesis holds that the expression of MHC class II genes in an inappropriate tissue could elicit an autoimmune response and destruction of this cell type. We will test this hypothesis by targeting the expression of the I-E antigen into beta cells of pancreas using insulin enhancer/promoter elements. We will also examine the function of MHC class II genes on the molecular level: an Ia molecule that has been altered at a potential Ia-antigen interaction site by site-directed mutagenesis will be tested for its function in presenting antigen to auto-, allo-, and antigen-specific T cell clones after transfection into a B cell line. The proposed studies should lead to a better understanding of the immune response phenomenon which is essential in order to gain more insight into possible mechanisms of immune disorders associated with human diseases such as some cancers, AIDS and autoimmune disorders.